1. Field of the Invention
The present invention relates to a length adjusting device for fastening straps of various masks, and in particular a length adjusting device suitable for a dustproof mask in which decreasing a gap between the mask and its wearer's face is required for preventing intrusion of harmful materials such as dust.
2. Prior Art
A conventional mask of this type is, as shown in FIGS. 10 and 11, provided with a mechanism for adjusting the length of fastening straps 12 at fitting portions of the straps 12 to the mask 11. This adjusting mechanism includes extruding pieces 13 respectively provided at upper and lower portions on both lateral sides of an edge portion of the mask 11, each of which has a through hole 16 with a notch 15 continued from a guiding groove 14, and a stopper 17 with an inlet and an outlet 17a on both sides, the stopper 17 bridging over the notch 15. A tip portion of the strap 12 of the mask 11 is inserted into the through hole 16 from thereunder, stretched over the stopper 17, into the inlet and outlet 17a thereof in the direction from the guiding groove 14 along the notch 15 and back again into the through hole 16, so that the fastening strap 12 is kept there immovably by the stopper 17. The length of the fastening strap 12 of the mask 11 is adjusted by the extent of insertion of the tip portion of the fastening strap 12 which is inserted through the through hole 16 from below.
Another known mask of this type that is capable of adjusting the length of straps includes a strap 12 provided with an engaging member 18 as shown in FIG. 12. Referring to FIG. 13, this engaging member 18 includes retaining holes 19 through which the fastening strap 12 of the mask 11 is inserted and by which the engaging member 18 is retained there, and an engaging portion 20 with which the other fastening strap 12' of the mask 11 is to be engaged. The engaging member 18 is fitted to the fastening strap 12 of the mask 11 that has passed through the retaining holes 19 before the fastening strap 12 is fixed to the extruding pieces 13 provided at the edge portion of the mask 11.
As stated above, conventional adjusting mechanisms for fastening straps of masks have complicated structures, are difficult to produce and entail rather high production costs, which are problems to be solved. Furthermore, the complicated structures cause the adjustment of the fastening straps 12 to a suitable length to be cumbersome. Therefore, even though the space between the mask 11 and a wearer's face could be narrowed, there still remains a problem to be solved; i.e. the straps 12 are likely to be fastened too tight or too loose and a comfortable fit is difficult to obtain.
It is further not sufficient to adjust the straps 12 to a suitable length by simply adding only the engaging member 18 to the adjusting mechanism for fastening straps of the mask. Moreover, since the engaging member 18 is designed to be fitted to one of the fastening straps 12 of the mask 11 in a manner that the strap 12 pierces through the retaining holes 19 of the member 18 before the fastening strap 12 is fixed to the extruding members 13 provided on the edge portion of the mask 11, the engaging member 18 cannot be removed from the fastening strap 12 readily even when the member 18 is not necessary.